In the development of safe-and-arm (S&A) devices for the implementation of safety in the use of explosive devices, the "unique-code device" (UCD) concept has been developed by the ordnance dicipline. A UCD is a device which is, in essence, an electrically operated combination lock. This device is of such a design that no plausable abnormal environments could cause the proper signals or dispacements which could cause arming of the controlled ordnance device. This is normally done using a complex system of cams, ratchets, pawls, levers, and electromagnets. Such devices are, of course, very precision in manufacture to meet military reliability requirements, and are quite difficult to design and develop. Most such systems are very costly and have marginal reliability.
It would be quite preferred to decrease the mechanical complexity of the UCD to enhance the cost/reliability factors if the unique-coding can be retained. A candidate for such a device is the multi-phase DC stepping motor. This device is commonly used in positioning and movement in precision devices such as computer printers and is very simple and economical in design. With a proper phase-relationship input from a computer circuit or other switching means, the motor will turn in precise angular increments due to its basic design. If the phase-relationships are at other than the proper value, the action of the motor is perfectly predictable due to the interaction of the magnetic fields and pole-pieces within the motor. With proper mechanical stops installed, and the requirement of at least one full turn of the motor shaft for arming, the probability for erronious arming is 1 in 65,000,000 for the common four-phase stepping motor.
A possible problem with the use of only a stepping motor is the case of overdriving the stepping motor. In this instance, it may under some conditions, with some motors, be possible to cause the motor to take more than one increment at a time. This may be possible if the stored rotational inertia in the motor armature is comparable in force to the restraining holding torque value of the motor. The rotational inertia of the armature is proportional to the square of its rotational speed, thus if the motor is constrained to operate at low speeds--i.e., slow stepping rates--there will be insufficient stored torque in the armature to cause overrunning of the magnetic holding field. Implementation of this constraining method would be to use magnetic damping.
It is an object of the present invention therefore, to provide a device for arming a safe-and-arm mechanism only after valid commands have been issued to the device.
It is a further object of the present invention to proivide such a device in which magnetic damping is achieved by eddy current braking.